During operations for working and converting long metallurgical products, it may be necessary to heat the product advancing through an installation such as a rolling mill which works and converts the product. This heating may be necessary in particular when it is desired to treat the product thermomechanically at the rolling heat or to provide a supply of heat allowing a given level of enthalpy of the product to be obtained, for example before it is shaped.
This heating may be obtained conventionally by exposing the surface of the product to one or more heating flames produced by one or more burners disposed on the path of the advancing product.
It is also known, in particular in the case of long products with circular cross section, to use inductors in the form of solenoids which include one or more windings in which the product is passed, in the direction of the axis of the windings which coincides with the axis of the long product. The windings of the inductor are supplied with alternating current and subject the product to a magnetic flux passing through the product in the axial direction and generating induced currents in the cross section of the product. Thus, that part of the long product which is advancing through the inductor is heated by induction, this heating method having the advantage of having great flexibility in use and high speed as well as a satisfactory thermal efficiency.
However, the use of an inductor in the form of a solenoid has certain drawbacks. First, the advancing product is surrounded by the inductor, so that it is very difficult if not impossible to adjust the relative positioning of the product with respect to the inductor, unless a solenoid is used whose internal diameter is very substantially greater than the diameter of the product. It is obvious that, in this case, a large decrease in the efficiency of the inductor and therefore in the maximum power which can be transmitted to the product must be accepted.
Furthermore, the engaging of the product in the inductor or in the successive inductors disposed on its route, present difficulties which are due to the small clearance existing between the product and the internal surface of the solenoid which is necessary in order to obtain sufficient efficiency and for the high speed of advance of the product, in particular when the heating is performed on the product at the output of a mill train.
In the case where the product or semifinished product being converted has shape defects, for example deformations in the shape of ski tips at its ends or undulations, it is not possible to modify the passage cross section of the solenoid in order to allow these deformed parts to pass, so that it is necessary to remove the inductor from the zone of passage of the product and to interrupt the heating, in order to avoid incidents which might have disastrous consequences.
In the case of long products having a cross section of non-circular shape and for example in the case of blooms or billets having a square-shaped cross section, inductors in the form of solenoids do not allow simultaneous provision of homogeneous heating of the product over its whole cross section and in particular in the vicinity of the corners of the square cross section and provision of heating at a high specific power.
Finally, in order to obtain heating of a sufficient amplitude, it may be necessary to use installations whose length determined by the length of the solenoids may be large. Furthermore, these relatively bulky installations are limited as to the maximum power which they can transmit to the product.
In the case of heating sheet metal edges on strip mill trains, it has been proposed to use a C-shaped inductor disposed laterally with respect to the advancing sheet metal, so that the poles of this inductor are placed on either side of the sheet metal, in the vicinity of the edge which is to be heated. This edge is passed through by a magnetic flux which is substantially perpendicular to the sheet metal which induces currents which have a high intensity, in particular in the vicinity of the external margin of the sheet metal. Such a method employing a magnetic flux which is transverse with respect to the edge is not, however, applicable to providing homogeneous heating of a long product over its whole cross section and to producing localised heating of this product in given zones such as the corners of a polygonal cross section.